Fluorescent dyes

ABSTRACT

Fluorescent dyes having the formula    &lt;IMAGE&gt;  [I]  wherein R1 represents hydrogen, (C1-C8) alkyl, trifluoromethyl, (C1-C4) alkoxycarbonyl, (C1-C4) alkoxycarbonyl (C1-C4) alkyl, phenyl, or aralkyl group; R2 represents hydrogen, cyano, acyl, (C1-C8) alkoxycarbonyl, (C1-C4) alkoxy (C1-C4) alkoxycarbonyl, (C1-C4) alkoxy (C1-C4) alkoxy (C1-C4) alkoxycarbonyl, di(C1-C4) alkylamino (C1-C4) alkoxycarbonyl, cyclohexyloxycarbonyl, aryloxycarbonyl, benzyloxycarbonyl, tetrahydrofurfuryloxycarbonyl, carbamoyl, N-(C1-C4) alkylcarbamoyl, N,N-di(C1-C4)alkylcarbamoyl, N-phenylcarbamoyl, or morpholinocarbonyl group; and R3 and R4 can be the same or different and each represents hydrogen, (C1-C8) alkyl, (C1-C4) alkoxy(C1-C4) alkyl, aralkyl or phenyl group which can be substituted by halogen atoms, (C1-C4) alkyl groups, or (C1-C4) alkoxy groups.

BACKGROUND OF THE INVENTION

The present invention relates to heterocyclic compounds which are fluorescent dyes. More particularly, it relates to fluorescent dyes which are used for coloring organic high polymers such as synthetic fibers e.g. polyester fiber in fluorescent brilliant yellow having high fastnesses.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel heterocyclic compounds which are fluorescent dyes having high light-fastness and sublimation fastness.

The heterocyclic compounds as the fluorescent dyes are novel compounds having the formula ##STR2## wherein R¹ represents hydrogen, (C₁ -C₈)alkyl, trifluoromethyl, (C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxycarbonyl(C₁ -C₄)alkyl, phenyl, or aralkyl group; R² represents hydrogen, cyano, acyl, (C₁ -C₈)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, di(C₁ -C₄)alkylamino(C₁ -C₄)alkoxycarbonyl, cyclohexyloxycarbonyl, aryloxycarbonyl, benzyloxycarbonyl, tetrahydrofurfuryloxycarbonyl, carbamoyl, N-(C₁ -C₄)alkylcarbamoyl, N,N-di(C₁ -C₄)alkylcarbamoyl, N-phenylcarbamoyl, or morpholinocarbonyl group; and R³ and R⁴ can be the same or different and each represents hydrogen, (C₁ -C₈)alkyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkyl, aralkyl or phenyl group which can be substituted by halogen atoms, (C₁ -C₄)alkyl groups, or (C₁ -C₄) alkoxy groups.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Suitable heterocyclic compounds include the compounds having the formula [I] wherein R¹ is hydrogen atom; (C₁ -C₈)alkyl group such as methyl and ethyl groups; (C₁ -C₄)alkoxycarbonyl group such as methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl and butoxycarbonyl groups; (C₁ -C₄)alkoxycarbonyl(C₁ -C₄)alkyl group such as methoxycarbonylmethyl, ethoxycarbonylmethyl, propoxycarbonylmethyl and butoxycarbonylmethyl groups; and aralkyl group such as benzyl and phenethyl groups; phenyl group and trifluoromethyl group; R² is hydrogen atom; cyano group; acyl group such as acetyl and benzoyl groups; (C₁ -C₈)alkoxycarbonyl group such as methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl and butoxycarbonyl groups; (C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl group such as methoxyethoxycarbonyl and ethoxyethoxycarbonyl groups; (C₁ -C₄)alkoxy(C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl group such as methoxyethoxyethoxycarbonyl, ethoxyethoxyethoxycarbonyl and propoxyethoxyethoxycarbonyl groups; di(C₁ -C₄)alkylamino(C₁ -C₄)alkoxycarbonyl group such as dimethylaminoethoxycarbonyl and diethylaminoethoxycarbonyl groups; cyclohexyloxycarbonyl group; aryloxycarbonyl group such as tolyloxycarbonyl and phenoxycarbonyl groups; benzyloxycarbonyl group; tetrahydrofurfuryloxycarbonyl group; carbamoyl group; N-(C₁ -C₄)alkylcarbamoyl such as methylcarbamoyl, ethylcarbamoyl, propylcarbamoyl and butylcarbamoyl groups; N,N-di(C₁ -C₄)alkylcarbamoyl group such as dimethylcarbamoyl and diethylcarbamoyl groups; N-phenylcarbamoyl group; and morpholinocarbonyl group; and R³ and R⁴ are respectively hydrogen atom; (C₁ -C₈)alkyl group such as methyl, ethyl, propyl and butyl groups; (C₁ -C₄)alkoxy(C₁ -C₄)alkyl group such as methoxyethyl, ethoxyethyl, methoxypropyl and ethoxypropyl groups; aralkyl group such as benzyl and phenethyl groups; and phenyl group which can be substituted by halogen atoms, (C₁ -C₄)alkyl groups or (C₁ -C₄)alkoxy groups such as phenyl, p-methoxyphenyl, p-tolyl and p-chlorophenyl groups.

The heterocyclic compounds having the formula [I] can be produced by condensing a compound having the formula ##STR3## wherein R¹ and R² are defined above, and X represents a halogen atom, with a compound having the formula ##STR4## wherein R³ and R⁴ are defined above, to form a perimidone ring.

The heterocyclic compounds having the formula [I] can be also produced by reacting a compound having the formula ##STR5## wherein R¹, R², R³ and R⁴ are defined above, with a carbonylating agent such as phosgene and methyl chloroformate to form a perimidone ring.

The former process is preferable.

The heterocyclic compounds having the formula [I] wherein R² is hydrogen atom, can be produced by hydrolyzing the ester part of the compound having the formula ##STR6## wherein R¹, R³ and R⁴ are defined above and R⁵ represents an alkyl group, to perform a decarboxylation.

The heterocyclic compounds having the formula [I] wherein R² is hydrogen atom, can be also produced by reacting a compound having the formula ##STR7## wherein R¹, R⁵ and X are defined above, with an urea compound having the formula

    R.sup.3 --NH--CONHR.sup.4                                  [VII]

wherein R³ and R⁴ are defined above to perform a hydrolysis and a decarbonylation in one step, to form a perimidone ring.

In order to effectively perform the hydrolysis and the decarbonylation, it is preferable to use the compounds having the formula [V] and [VI] wherein R⁵ is a lower alkyl group such as methyl and ethyl groups.

The process for producing the heterocyclic compounds having the formula [I] will be further illustrated.

The reaction of the compound having the formula [II] with more than equimole of the urea compound having the formula [III] is carried out in an inert organic solvent such as chlorobenzene, O-dichlorobenzene, trichlorobenzene, nitrobenzene, O-nitrotoluene, diphenyl, diphenyl ether, N,N-dimethyl formamide, N-methylpyrrolidone, dimethyl sulfoxide, diphenyl ethane, quinoline, and alkylnaphthalene, at 100° to 250° C. preferably 150° to 230° C. The heterocyclic compound having the formula [I] can be isolated from the reaction mixture and purified it by the conventional manner.

The compound having the formula [II] can be produced as follows.

4,5-Dihalogenonaphthalic acid-N-aminoimide having the formula ##STR8## wherein X represents a halogen atom is reacted with a carbonyl compound having the formula

    R.sup.1 --COCH.sub.2 --R.sup.2                             [IX]

wherein R¹ and R² are defined above, and the resulting hydrazone compound having the formula ##STR9## wherein R¹ and R² are defined above, is reacted with an alkaline condensing agent to form a cyclization.

The carbonyl compounds having the formula [IX] can be acetoacetic esters; phenyl acetoacetic esters; benzoyl acetoacetic esters; oxalacetic esters; 1,3-dicarbonyl compounds e.g. benzoyl acetone, acetyl acetone, dibenzoyl methane, formyl acetophenone and 2-thenoyltrifluoroacetone; acetone dicarboxylic acid diesters; acetoacetic acid amides and benzoyl acetonitrile.

In the reaction, 4,5-halogenonaphthalic-N-aminoimide having the formula [VIII] and at least equivalent of the carbonyl compound having the formula [IX] are heated in an inert organic solvent such as ethanol, isobutanol, monomethylglycol, acetic acid, chlorobenzene, dichlorobenzene, trichlorobenzene, nitrobenzene, O-nitrotoluene, N,N-dimethylformamide, or N-methylpyrrolidone, or in excess of the carbonyl compound having the formula [IX] in the presence of an acidic compound such as benzenesulfonic acid, toluenesulfonic acid, ethanesulfonic acid, aminosulfonic acid, sulfuric acid, phosphoric acid and hydrochloric acid at 80° to 200° C., and the reaction mixture is cooled to room temperature and is diluted with methanol, acetic acid or water, whereby the hydrazone compound having the formula [X] is isolated as crystals.

The resulting hydrazone compound is heated in an inert organic solvent such as butanol, isobutanol, pyridine, dimethyl sulfoxide, N,N-dimethyl formamide, dimethyl acetamide, hexamethyl phosphoric triamide and N-methylpyrrolidone in the presence of an alkaline compound such as sodium acetate, potassium acetate, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, piperidine, morpholine and triethylamine at 80° to 200° C. and the reaction mixture is cooled to room temperature and diluted with methanol, acetic acid or water to isolate 3,4-dihalogeno-7H-benzo[de]pyrazolone[5,1-a]isoquinoline-7-on derivative having the formula [II] as crystals.

The organic high polymers which are dyed with the heterocyclic dyes of the present invention can be fibers, films, sheets and other shaped articles made of a synthetic polymer or an artifical polymer such as polyesters, polyesterethers, polyamides, polyurethanes, polyacrylonitriles, polyolefins, polyvinyl chlorides, polystyrenes, polyacrylates, polymethacrylates and cellulose acetates; mixed spun fibers of synthetic fiber or artificial fiber such as polyester fiber, polyamide fiber and cellulose acetate fiber; and mixed spun fibers of the synthetic fiber or the artificial fiber with natural fiber such as cellulose and wool; and shaped articles made of a copolymer such as AS resins and ABS resins.

These organic high polymers can be dyed with the heterocyclic dye having the formula [I] by the conventional manners.

When the fiber is dyed, the heterocyclic dye having the formula [I] is dispersed in an aqueous medium by the conventional manner with a dispersing agent such as condensates of naphthalenesulfonic acid and formaldehyde, higher alcohol sulfates and higher alkylbenzenesulfonates to prepare a dyeing bath or printing paste and the fiber is dyed by the dip dyeing or the printing.

In the dip dyeing, the fiber can be dyed in high fastnesses by applying the conventional dyeing process of a dispersing dye such as the high temperature dyeing process, the carrier dyeing process and the thermosol process.

The synthetic fibers such as polyester fiber can be colored by the dope dyeing process by adding the heterocyclic dye having the formula [I] at the polymerization or the melt-spinning.

When the synthetic resins are colored, the heterocyclic dye having the formula [I] is added to the synthetic resin and the mixture is heat-blended to mold by the press molding process, the injection molding process, the calender molding process and the extrusion molding process or the heterocyclic dye having the formula [I] is added to a liquid monomer or prepolymer and the mixture is polymerized or cured by the cast molding process whereby the colored article having high fastnesses can be obtained.

The present invention will be further illustrated by certain examples which are provided for purposes of illustration only and not intended to be limiting unless otherwise specified.

EXAMPLE 1 ##STR10##

A dyeing bath was prepared by dispersing 0.5 g of the compound having the above-mentioned formula in 3 liters of water containing 1 g of a condensate of naphthalenesulfonic acid and formaldehyde and 2 g of higher alcoholsulfate.

A dip dyeing was carried out by dipping 100 g of polyester fiber in the dyeing bath at 130° C. for 60 minutes, soaping, washing with water and drying it to obtain polyester fiber having fluorescent brilliant greenish yellow color in high light fastness and sublimation fastness.

The perinone type compound used in the example was produced by the following process. ##STR11##

The reaction was carried out by heating 18 g of the compound having above-mentioned formula and 57 g of N,N'-dimethyl urea in 200 ml of nitrobenzene under refluxing for 5 hours. After cooling the reaction mixture, it was diluted with methanol, and the precipitate was filtered and sequentially washed with methanol and with water and dried to obtain 9 g of yellow crystals. The product was recrystallized from dimethyl formamide to obtain yellow crystals having a melting point of 320° to 321° C.

    ______________________________________                                         Elemental Analysis:                                                                         C %     H %       N %                                             ______________________________________                                         Calculated     71.08     4.30      13.26                                       Found          70.91     4.15      13.11                                       ______________________________________                                          ##STR12##                                                                

EXAMPLE 2 ##STR13##

A dyeing bath was prepared by dispersing 0.5 g of the compound having the above-mentioned formula in 3 liters of water containing 1.5 g of a condensate of naphthalenesulfonic acid and formaldehyde and 1 g of higher alcoholsulfate and adding 15 g of a methyl naphthalene type carrier.

A dip dyeing was carried out by dipping 100 g of polyester fiber in the dyeing bath at 100° C. for 90° C., soaping, washing with water and drying it to obtain polyester fiber having fluorescent brilliant greenish yellow color in high fastnesses.

The compound used in the example was in a form of yellow crystals having a melting point of 305° to 307° C.

The compound was produced by the perimidonation in accordance with the process of Example 1 except substituting the compound having the formula ##STR14## with the compound having the formula ##STR15##

    ______________________________________                                         Elementaly Analysis:                                                                        C %     H %       N %                                             ______________________________________                                         Calculated     64.60     4.65      14.35                                       Found          64.35     4.42      14.12                                       ______________________________________                                          ##STR16##                                                                

EXAMPLE 3

In accordance with the process of Example 1 except using the compounds shown in Table 1, polyester fiber was dyed to obtain the dyed polyester fibers having the color shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________      ##STR17##                                                                     __________________________________________________________________________     No.          1            2        3        4                                  __________________________________________________________________________     R.sup.1      CH.sub.3     CH.sub.3 CH.sub.3 CH.sub.3                           R.sup.2                                                                                      ##STR18##   COOC.sub.2 H.sub.5                                                                      COOC.sub.4 H.sub.9 (iso)                                                                 ##STR19##                         R.sup.3      CH.sub.3     CH.sub.3 CH.sub.3 C.sub.2 H.sub.5                    R.sup.4      CH.sub.3     CH.sub.3 CH.sub.3 C.sub.2 H.sub.5                    mp. (°C.)                                                                            320-321      305-307  339-340  235-237                            Mass spectrum                                                                               422          390      418      450                                 ##STR20##                                                                     Elemental analysis:                                                                  Cal.   71.08        64.60    66.02    71.99                              C (%) Found  70.91        64.35    65.76    72.23                                    Cal.   4.30         4.65     5.30     4.92                               H (%) Found  4.15         4.42     5.31     5.08                                     Cal.   13.26        14.35    13.29    12.44                              N (%) Found  13.11        14.12    13.29    12.28                              __________________________________________________________________________     Color        Bril.        Bril.     Bril.   Bril.                              dyed polyester                                                                              G-Y          G-Y       G-Y     G-Y                                __________________________________________________________________________     No.          5            6         7       8                                  __________________________________________________________________________     R.sup.1                                                                                      ##STR21##   CH.sub.3  CH.sub.3                                                                               CH.sub.3                           R.sup.2                                                                                      ##STR22##   H         H       COOC.sub.4 H.sub.9 (iso)           R.sup.3      CH.sub.3     CH.sub.3  C.sub.2 H.sub.5                                                                        C.sub.3 H.sub.7 (n)                R.sup.4      CH.sub.3     CH.sub.3  C.sub.2 H.sub.5                                                                        C.sub.3 H.sub.7 (n)                mp. (°C.)                                                                            354-355      352-356   292-294 186-188                            Mass spectrum                                                                               484          318       346     474                                 ##STR23##                                                                     Elemental analysis:                                                                  Cal.   74.22        67.92    69.35    68.33                              C (%) Found  74.02        67.68    69.18    68.22                                    Cal.   4.15         4.43     5.24     6.37                               H (%) Found  4.18         4.53     5.26     6.31                                     Cal.   11.54        17.60    16.17    11.81                              N (%) Found  11.62        17.98    15.98    11.76                              __________________________________________________________________________     Color        Bril.        Bril.    Bril.    Bril.                              dyed polyester                                                                              G-Y          G-Y      G-Y      G-Y                                __________________________________________________________________________     No.           9              10             11                                 __________________________________________________________________________     R.sup.1       CH.sub.3       CH.sub.3       CH.sub.3                           R.sup.2       COOC.sub.4 H.sub.9 (iso)                                                                      COOC.sub.4 H.sub.9 (iso)                                                                      COOC.sub.4 H.sub.9 (iso)           R.sup.3       C.sub.4 H.sub.9 (n)                                                                           (CH.sub.2).sub.3 OCH.sub.3                                                                    C.sub.2 H.sub.5                    R.sup.4       C.sub.4 H.sub.9 (n)                                                                           (CH.sub.2).sub.3 OCH.sub.3                                                                    C.sub.2 H.sub.5                    mp. (°C.)                                                                             199-201        165-170        270-273                            Mass spectrum 502            534            446                                 ##STR24##                                                                     Elemental analysis:                                                                  Cal.    69.30          65.15          67.25                              C (%) Found   69.21          65.10          67.21                                    Cal.    6.82           6.41           5.87                               H (%) Found   6.78           6.38           5.82                                     Cal.    11.15          10.48          12.55                              N (%) Found   11.06          10.41          12.51                              __________________________________________________________________________     Color         Bril.          Bril.          Bril.                              dyed polyester                                                                               G-Y            G-Y            G-Y                                __________________________________________________________________________     No.           12            13           14                                    __________________________________________________________________________     R.sup.1       CH.sub.3      CH.sub.3     CH.sub.3                              R.sup.2                                                                                       ##STR25##                                                                                    ##STR26##   COOC.sub.2 H.sub.4 OC.sub.4                                                    H.sub.9 (n)                           R.sup.3       CH.sub.3      CH.sub.3     CH.sub.3                              R.sup.4       CH.sub.3      CH.sub.3     CH.sub.3                              mp. (°C.)                                                                             280-288       327-328      261-262                               Mass spectrum 446           444          462                                    ##STR27##                                                                     Elemental analysis:                                                                  Cal.    64.56          67.55          64.92                              C (%) Found   64.39          67.51          64.88                                    Cal.    4.97           5.44           5.67                               H (%) Found   4.81           5.41           5.62                                     Cal.    12.55          12.61          12.12                              N (%) Found   12.44          12.58          12.08                              __________________________________________________________________________     Color         Bri.           Bril.          Bril.                              dyed polyester                                                                               G-Y            G-Y            G-Y                                __________________________________________________________________________     No.           15             16             17                                 __________________________________________________________________________     R.sup.1       CH.sub.3       CH.sub.3       CH.sub.3                           R.sup.2       COO(C.sub.2 H.sub.4 O).sub.2 C.sub.2 H.sub.5                                                   ##STR28##     COOC.sub.6 H.sub.13 (n)            R.sup.3       CH.sub.3       CH.sub.3       CH.sub.3                           R.sup.4       CH.sub.3       CH.sub.3       CH.sub.3                           mp. (°C.)                                                                             242-246        220-225        289-290                            Mass spectrum 478            452            446                                 ##STR29##                                                                     Elemental analysis:                                                                  Cal.    62.75          69.01          67.25                              C (%) Found   62.71          68.95          67.18                                    Cal.    5.48           4.46           5.87                               H (%) Found   5.41           4.41           5.81                                     Cal.    11.71          12.38          12.55                              N (%) Found   11.68          12.31          12.47                              __________________________________________________________________________     Color         Bril.          Bril.          Bril.                              dyed polyester                                                                               G-Y            G-Y            G-Y                                __________________________________________________________________________      Note:-                                                                         Bril. : brilliant                                                              G-Y : greenish yellow                                                     

What is claimed is:
 1. A heterocyclic dye having the formula: ##STR30## wherein R¹ represents hydrogen, (C₁ -C₈)alkyl, trifluoromethyl, (C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxycarbonyl(C₁ -C₄)alkyl, phenyl, benzyl or phenethyl; R² represents hydrogen, cyano, acetyl, benzoyl, (C₁ -C₈)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, di(C₁ -C₄)alkylamino(C₁ -C₄)alkoxycarbonyl, cyclohexyloxycarbonyl, tolyloxycarbonyl, phenoxycarbonyl, benzyloxycarbonyl, tetrahydrofurfuryloxycarbonyl, carbamoyl, N-(C₁ -C₄)-alkylcarbamoyl, N,N-di(C₁ -C₄)alkylcarbamoyl, N-phenylcarbamoyl, or morpholinocarbonyl; and R³ and R⁴ can be the same or different and each represents hydrogen, (C₁ -C₈)alkyl, (C₁ -C₄)-alkoxy(C₁ -C₄)alkyl, benzyl, phenethyl or phenyl which can be substituted by halogen atoms, (C₁ -C₄)alkyl groups, or (C₁ -C₄)alkoxy groups.
 2. The heterocyclic dye of claim 1, wherein R¹ represents hydrogen, (C₁ -C₄)alkyl, or phenyl; R² represents hydrogen, acetyl, benzoyl, (C₁ -C₄)alkoxycarbonyl, or (C₁ -C₄)alkoxyethoxycarbonyl; and R³ and R⁴ can be the same or different and each represents (C₁ -C₄)alkyl, (C₁ -C₄)alkoxyalkyl, or phenyl.
 3. The heterocyclic dye of claim 1, wherein R¹ represents hydrogen, methyl, or ethyl; R² represents hydrogen, acetyl, benzoyl, methoxycarbonyl, ethoxycarbonyl, n-butyloxycarbonyl, isobutyloxycarbonyl, methoxyethoxycarbonyl, ethoxyethoxycarbonyl, n-butyloxyethoxycarbonyl, or isobutyloxyethoxycarbonyl; R³ represents methyl or ethyl; and R⁴ represents methyl or ethyl.
 4. A heterocyclic dye having the formula: ##STR31## wherein R¹ represents hydrogen, (C₁ -C₈)alkyl, trifluoromethyl, (C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxycarbonyl(C₁ -C₄)alkyl, phenyl, benzyl or phenethyl; R² represents cyano, acetyl, benzoyl, (C₁ -C₈)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, (C₁ -C₄)alkoxy(C₁ -C₄)alkoxy(C₁ -C₄)alkoxycarbonyl, di(C₁ -C₄)alkylamino(C₁ -C₄)alkoxycarbonyl, cyclohexyloxycarbonyl, tolyloxycarbonyl, phenoxycarbonyl, benzyloxycarbonyl, tetrahydrofurfuryloxycarbonyl, carbamoyl, N-(C₁ -C₄)alkylcarbamoyl, N,N-di(C₁ -C₄)alkylcarbamoyl, N-phenylcarbamoyl, or morpholinocarbonyl; and R³ and R⁴ can be the same or different and each represents hydrogen, (C₁ -C₈)alkyl, (C₁ -C₄)alkoxy(C₁ -C₄)-alkyl, benzyl, phenethyl, or phenyl which can be substituted by halogen atoms, (C₁ -C₄)alkyl groups, or (C₁ -C₄)alkoxy groups.
 5. The heterocyclic dye of claim 4, wherein R¹ represents hydrogen, (C₁ -C₄)alkyl, or phenyl;R² represents acetyl, benzoyl, (C₁ -C₄)alkoxycarbonyl, or (C₁ -C₄)alkoxyethoxycarbonyl; and R³ and R⁴ can be the same or different and each represents (C₁ -C₄)alkyl, (C₁ -C₄)alkoxyalkyl, or phenyl.
 6. The heterocyclic dye of claim 4, wherein R¹ represents hydrogen, methyl, or ethyl; R² represents acetyl, benzoyl, methoxycarbonyl, ethoxycarbonyl, n-butyloxycarbonyl, isobutyloxycarbonyl, methoxyethoxycarbonyl, ethoxyethoxycarbonyl, n-butyloxyethoxycarbonyl, or isobutyloxyethoxycarbonyl; R³ represents methyl or ethyl; and R⁴ represents methyl or ethyl. 